1. Field of the Invention
This invention relates to feeds for transmission lines, and concerns in particular methods of and apparatus for supplying a coaxial cable transmission line with microwave energy in circularly symmetric, TEM, mode.
2. Description of the Prior Art
There are many occasions in the field of microwave electronics when it is desired to transfer microwave energy into a coaxial cable transmission line in circularly symmetric (Transverse Electromagnetic) mode. One such occasion that frequently occurs is in low power radar systems, wherein rotating joints are employed in the transfer of microwave electromagnetic radiation energy between two sections of apparatus one of which rotates relative to the other, as now explained in more detail.
It is common, in microwave transceiving apparatus of the type used in a radar system, for the signals to be transmitted to be transferred from one or more microwave signal generators/transmitters to a physically separate aerial from which they are to be radiated. In many radar systems the aerial may be required to rotate about a vertical and/or horizontal axis, so as to radiate the energy in a succession of different directions, and in some of these systems--particularly those where the aerial rotates continuously in one direction--it is necessary to transfer the microwave energy via the mechanical joint by which the aerial is mounted for rotation upon a base portion stationary relative to the ground. In a radar employing fairly low energy microwaves in the centimetric/metric wavelength range (say, in the L to C band range, or from 30 cm down to 6 cm) the energy will commonly be transferred along coaxial cables (rather than waveguides) and in such a case the joint is conveniently constructed as a series of coaxial tubular conductors, each constituting a physically separate channel, sufficient in number to enable each conductor to carry one of the signals to be transferred. With this construction, the innermost joint conductor (a first channel) is connected to the centre conductor of the first one of the coaxial cables feeding the joint (so "continuing" the cable through the joint), while the next joint conductor (a second channel) is connected both to the outer conductor of that first cable and to the inner conductor of a second cable, the next joint conductor (a third channel) is connected both to the outer conductor of the second cable and to the inner conductor of the third cable . . . and so on until the outermost joint conductor is connected only to the outer conductor of the final cable.
One acceptable way of feeding the microwave energy to the joint conductors (other than the innermost) is the well-known "stub-supported" fashion. In this, a laterally-extending stub of the relevant conductor is positioned along the conductor about .lambda./4 (where .lambda. is the mean free-space wavelength of the desired signal bandwidth) from a short to the relevant outer conductor, and the energy is supplied to the conductor via the stub (the .lambda./4-spaced short "supports" the stub, assisting in the proper launching of energy along the conductor). There are, however, problems associated with this arrangement, as is now explained.
When microwave energy is transferred between the stationary and rotating sections of a tubular rotating joint it is important that the relative angular position of the two joint sections play no part in determining how much energy is transferred. This can be achieved by arranging that the energy distribution around the tube be uniform (so that there is no angular dependency at the moment of transfer), and this uniformity, or circular symmetry, describes the pure transverse electromagnetic mode--TEM--of energy propagation. Unfortunately, the presently-preferred method of feeding the energy to the tubular conductor--thus, using a stub--is inherently asymmetric, and guarantees that a component of the resulting field will itself propagate asymmetrically. Luckily, this proportion is low, and decays rapidly (in an exponential manner), but even so to prevent its transfer across the joint the length of conductor between the input and output stubs must be relatively large to assure the asymmetric component's decay to an acceptably low level before the transferred energy is launched into the line fed by the output side of the joint.
This need to have a significant length of conductor between the two stubs in a stub-supported joint, coupled indeed with the .lambda./4 length of conductor beyond the stub at each end, means that stub-supported joints are considerably longer than is desirable bearing in mind the general need always to have the equipment occupy the smallest possible space. The invention seeks to deal with this length problem by utilizing a different, and novel, method of and apparatus for supplying the energy to each joint conductor--which is indeed applicable to the launching of microwave energy into any coaxial cable transmission line--where there is employed a conductive feed ring (or short tube) positioned around and spaced from the conductor, and this ring is itself fed by a stub and is shorted to the relevant outer joint conductor at a point diametrically opposite the stub.